The ability of Grammostola. mollicoma (Ausserer 1875) spiderlings
(Araneae, Theraphosidae) to emerge from the cocoon without the
assistance of their mother was tested experimentally. We created two
experimental groups with 23 cocoons in each group. In one of the groups
we cut the cocoon wall creating an opening; in the other group, the
cocoon remained untouched. We found no differences between the groups in
either the number or instar composition of the spiderlings that emerged.
The spiderlings were able to emerge without the assistance of their
mother. The emerging instars in both groups were precocious compared to
previous suggestions in the literature.

Once spiders hatch from their eggs, they usually molt one or more
times inside the cocoon or egg sac before they emerge. Although cocoon
care and postembryonic development in spiders have been thoroughly
studied, spiderling emergence is only known from a few observations,
mainly in Araneomorphae (Eason 1964; Engelhard 1964; Fujii 1978; Vannini
et al. 1986; Riechert & Jones 2001; Kurpick 2000; Viera et al. 2007
a & b). The mother's assistance is indispensable for the
emergence of juveniles from the cocoon in lycosids (Fujii 1978; Higashi
& Rovner 1975), in the subsocial theridiid Anelosimus cf studiosus
(Viera et al. 2007b), and in the eresid Stegodyphus lineatus (Latreille
1817) (Schneider and Lubin 1997). In these species, spiderlings are
unable to open and exit the egg sac by themselves and die if the mother
does not open it. In contrast, in other Araneomorphae like orb weavers,
where the mother dies a few days after building the cocoon, the
spiderlings are capable of emerging even in her absence (Foelix 1996).

In mygalomorphs, Coyle & Icenogle (1994) suggested from
indirect evidence that spiderlings of the antrodiaetid Aliatypus spp.
may need the mother's assistance to emerge, while Marechal (1994)
observed that spiderlings of the diplurid Ischnotele guianensis
(Walckenaer 1837) are able to hatch by themselves from the cocoon
without external help. No other reports about spiderling emergence are
available for most mygalomorph families.

Usually the eclosion of the spider from the egg determines the
transition from the embryonic to postembryonic life (Foelix 1996).
However, the characteristics in which the spiders hatch varies in
different species (Holm 1940; Vachon 1957; Peck & Whitcomb 1970;
Ramousse & Wurdak 1984), contributing to a confusion in the terms
and descriptions of developmental instars in spiders (Foelix 1996). As
Galiano has provided a comprehensive study of theraphosid development
(1969, 1973a & b, 1984), we followed her terms and concepts. Using
her terms, instar A is the 1st intrachorional state and consequently
instar B is the first instar out ofthe egg. Although in instar B the
spider is completely free of the egg, the body is bent and the legs are
extended but do not contact with substratum and are not functional for
locomotion. In addition, the eyes and several kinds of setae are absent.
These instars as well as instars C and D are completed within the cocoon
in Grammostola pulchripes (Simon 1891). More intra-cocoon instars and
less development in the early instars was considered as a derived
characteristic for Theraphosidae in comparison with other mygalomorphs
(Galiano 1969).

In this study we experimentally tested if spiderlings of the
theraphosine G. mollicoma are able to emerge from cocoons without the
assistance of their mothers. We additionally compared the instars of
emergence with predictions from previous postembryonic studies (Galiano
1969, 1973), discussing their possible adaptive value.

METHODS

We used 46 cocoons of G. mollicoma (northern form) obtained through
the Uruguayan mail and confiscated from illegal trade. All were
presumably collected at a site near Achar, Tacuarembo, Uruguay
[32[degrees]23'60"S, 56[degrees]04'57"W] considering
police evidence, habitat description and known distribution of this
species. In addition, 800 adults of G. mollicoma were simultaneously
confiscated in another mailing by the same person from the same
locality.

Cocoons were maintained in plastic containers (83 mm diam. X 105 mm
high). Observations took place from 24 January to 7 March 2007. We
divided the cocoons into two experimental groups with 23 cocoons in each
one. In one of these groups (treatment group) we made a cut 5 mm long
with scissors in the cocoon wall on the first day of observation (this
cut was approximately of the same size as natural orifices made by
spiderlings); in the other group (control), the cocoons remained closed.
During the experimental period the room temperature varied between 26.6
[+ or -] 1.4[degrees] c and 24.2 [+ or -] 1.4[degrees] C, and
photoperiod was natural (approximately 14 h day:10 h night).

We examined the cocoons daily to monitor the
spiderlings'emergence. When the spiderlings emerged, we counted
them and determined the postembryonic instars (following Galiano 1969).
Instar A is the last intracorional instar and B is the first
extracorional instar; we did not observe these instars in our study.
Instar C characteristics include: bent body, absence of body pigments,
absence of eyes (only maculas), legs not completely functional. Instar D
has cephalothorax and abdomen in the same plane, pigments present, eyes
present, absence of tarsal scopulae and claw tufts, slow locomotion.
Instar E shows body densely hirsute, scopula and claw tufts present and
normal locomotion. When more than 50 spiderlings emerged, the remaining
progeny was preserved in ethanol and examined, the cocoon was measured
(major and minor axis) and its natural openings were counted. At the end
of the experiment, the cocoons that remained closed were opened and
examined. Voucher spiderlings and opened cocoons were deposited in the
arachnology collection of Facultad de Ciencias, Montevideo, Uruguay.

RESULTS

Spiderlings successfully emerged from 22 cocoons in the treatment
group and 19 in the control group (non-treatment). No significant
differences were found in the frequency of spiderling emergence between
groups ([chi square] = 2.02, P = 0.15). The mean number of spiderlings
born alive per cocoon in the treatment group was (mean [+ or -] SD) 91.5
[+ or -] 46.8 and in the control 102.0 6 38.4, again with no significant
differences between groups (t = 0.78, P = 0.45).

The spiderlings emerged from their cocoons in instars C, D, and E,
both in the treatment and control groups. Very few spiderlings emerged
in instar C: one and six from each of two cocoons of the treatment group
and one from a cocoon of the control group. Curiously, in some cocoons
spiderlings emerged simultaneously in two different instars (Table 1).
No additional teeth on the chelicerae nor bifurcated cheliceral tips or
other structures related to cocoon opening were found in the emerged
spiderlings. Table 1 shows how the pattern of instar emergence was
distributed among the cocoons. The distribution of mean numbers of
spiderlings emerged in instars D and E by cocoon group are shown in Fig.
1. All the spiderlings that emerged in instar D molted to instar E
within 24 h after emergence.

Of the four cocoons from which spiderlings did not emerge, we found
eggs infected with fungus in two of them, dry eggs in another, and nine
spiderlings alive (three of them molting), seven dead, and several
unhatched eggs in the last cocoon.

The period from the beginning of the observation to the emergence
of spiderlings from the cocoons took 12.9 [+ or -] 8.2 days in the
treatment group and 10.1 [+ or -] 5.9 days in the control group, with no
significant difference between these periods (t = 1.25, P = 0.23).

Cocoons averaged 43.3 [+ or -] 6.8 mm and 39.6 [+ or -] 7.4 mm
(major and minimum axes) in the treatment group and 43.8 [+ or -] 5.8 mm
and 41.6 [+ or -] 5.8 mm in the control group, showing no significant
differences in cocoon size between the groups (major axis: t = 0.28, P =
0.78; minimum axis: t = 0.96, P = 0.34). The number of natural
perforations, if we do not consider the experimental cut, was 0.55 [+ or
-] 0.60 in the treatment group and 1.53 [+ or -] 0.61 in the control
group. Significant differences were found between groups (t = 5.19; P
< 0.0001).

DISCUSSION

Our results clearly showed that spiderlings of G. mollicoma are
able to emerge from the cocoon without the assistance of their mother as
was reported by Marechal (1994) for the diplurid I. guianensis. Coyle
& Icenogle (1994) did not observe direct evidence for the
mother's assistance in the antrodiaetid Aliatypus spp. Their
indirect observations provided weak support for the hypothesis that the
mother's help is required. The small number of natural perforations
found in the cocoons of the treatment group suggests that spiderlings
could utilize pre-existent holes.

[FIGURE 1 OMITTED]

Copulation during egg sac care was reported recently for this
species (Postiglioni 2007). The ability of spiderlings to emerge without
assistance might be an important trait that permits the mother to be
receptive and exposed to the risks (i.e., predation) of courtship and
copulation without jeopardizing the success of her spiderlings.

The mean number of spiderlings per cocoon (or clutch) for G.
mollicoma was moderately low in comparison with most Theraphosidae
(Table 2). This characteristic could be interpreted as plesiomorphic if
compared with the sister family Barychelidae with 20-80 eggs (Raven
1994). Other theraphosids as Aphonopelma joshua Prentice 1997,
Plesiopelma longisternale (Schiapelli & Gerschman 1942), Theraphosa
blondi (Latreille 1804) and some avicularines share with G. mollicoma a
low clutch size.

Galiano (1969) observed a bifurcated tip on the cheliceral fang in
instars B and C of G. pulchripes (in synonymy with G. mollicoma by
Perez-Miles et al. 1996), which we found to be absent in instar C of G.
mollicoma. Galiano also indicated the presence of maxillary cuspules and
the scarcity of hairs on tarsi and tibiae in instar C of G. pulchripes
but we found no such cuspules and several tibial and tarsal hairs, which
causes us to question the synonymy of these species. The absence of
special structures in spiderlings related to cocoon opening suggests
that they open the cocoon with the chelicerae. The experimental
perforation seemed not to affect the normal development of the cocoons
nor the number of spiderlings born alive, considering the absence of
significant differences between the groups. Instar C seems to be a
pre-emergence instar because few individuals emerged in this instar, and
only in the treatment group. The absence of eyes and pigments and the
bent body, which impedes locomotion (Galiano 1969), also agree with a
pre-emergence instar.

Galiano (1969) incubated isolated eggs of G. pulchripes outside the
cocoon, and based on morphological evidence during development, proposed
that spiderlings emerge in instar E. Our results partially agree with
this author because we found that half of G. mollicoma spiderlings
emerged in instars D, molting to E outside the cocoon within about 24 h.
The emergence in two different instars also indicates a slight
asynchrony of molting between instars D and E. However, instar E seems
to be the first in which the spiderlings are independent enough and
ready for their free life (Galiano 1969). Instar E is the first instar
where urticating hairs develop (Perez-Miles 2002) and the spiderlings
are able to feed by themselves (occasional observations). It is not
clear what the advantages are for the spiders to emerge in the
precocious instar D. Probably in this instar spiderlings are not able to
open the cocoon by themselves and instead use the openings made by
spiderlings in instar E to emerge. Prentice (1997) reported that
spiderlings of Aphonopelma joshua Prentice 1997 emerge in the fourth or
fifth instar probably homologous with D and E, which is similar to our
findings in G. mollicoma.

Galiano (1969), following Holm (1956), indicated that a low
quantity of yolk and an increased degree of organization in early
extracorional instars imply a few number of instars intracocoon and a
plesimorphic condition, as in Telechoris striatipes (Simon 1889) [(ex
Ischnothele karschi (Bosenberg & Lenz 1895)]. Galiano (1969) has
stressed the importance of having four instars intra-cocoon in G.
mollicoma and consequently considered this characteristic as derived in
comparison with most mygalomorphs and other Araneae. For example the
Mesothelae Heptathela kimurai (Kishida 1920) and other mygalomorphs such
us Atypus karschi Donitz 1887 and Telechoris striatipes, have only two
or three states intra-cocoon (Yoshikura 1952, 1958 and Holm 1956; cited
by Galiano 1969). In H. kimurai and T. striatipes, spiderlings have two
instars inside the cocoon while A. karschi has three. Stradling (1994)
reported only one intra-cocoon postembryonic instar in Avicularia
avicularia (Linnaeus 1758), which seems to be a record in Theraphosidae.
Our findings in G. mollicoma with three instars intra-cocoon question
the indirect evidence of Galiano (1969) and consequently the
evolutionary interpretation of Galiano.

ACKNOWLEDGMENTS

We thank Anita Aisenberg, Fernando G. Costa and Carmen Viera for
their valuable comments on an early draft of this paper; and two
anonymous reviewers for their suggestions.

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